1. Field of the Invention
This invention relates to optical waveguide fibers, and more particularly, to the improvement of the fatigue characteristics of such fibers.
Static fatigue failure in an optical fiber manifests itself as a deteriorating load-carrying capability under a constant axial stress in a humid atmosphere. Glass has a known propensity to weaken with time and under conditions of mechanical stress because of water reaction at the tip of surface flaws. In the manufacture, installation and servicing of a glass optical fiber cable, the fibers experience unavoidable short-term and long-term stresses. Short-term stresses arise from pulling and handling the fibers during manufacturing and installation, or from accidentally applying severe curvatures or impacts. Long-term stresses arise from suspending a cable between supports, from residual stresses due to cabling, and from reeling tension when the cable is taken up and stored. The glass fibers in an optical fiber cable must of course be capable of withstanding such stresses.
The wide variation in the strength of glass is due to the existence of surface flaws. Depending on the prior processing history, surface flaws can range from atomic discontinuities to macroscopic cracks. For an optical fiber, the more surface flaw damage it incurs, the lower will be its tensile strength, and the more likely it is to fail completely under tensile stress.
Glass optical fibers subjected to long-term stress can fracture in a delayed fashion, even though the stress can be withstood for a short time. A method which provides the physical basis for extrapolating laboratory data to multi-kilometer lengths and long lifetimes is discussed in the publication by R. Olshansky and R. D. Maurer entitled "Tensile Strength and Fatigue of Optical Fibers" Journal of Applied Physics, Vol. 47, No. 10, October 1976, pp. 4497-4499. In accordance with this publication the growth in flaw size from water attacking the strained glass bonds at the flaw tip is described by an empirical law EQU (d.delta./dt)=(AY.delta..sup.1/2 .sigma.).sup.n ( 1)
where .delta. is the flaw depth, Y.about..sqroot..pi., .sigma. is the applied stress, and A and n are empirical constants. Equation (1) is dominated by n which is about 20 for conventional fibers under normal humidity. It is noted that the value of n depends upon the measurement technique employed. Since all of the data disclosed herein was obtained by the same measurement technique, valid comparisons can be made between the measured n values of the various fibers that were tested. The constant, n expresses the stress dependence of fatigue and hence is often used to compare fatigue susceptibility in different cases. Larger values of n indicate better resistance to fatigue. When the stress increases linearly with time, .sigma.=Rt, equation (1) predicts that the same flaw will fracture under different loading stress according to EQU (.sigma..sub.1 /.sigma..sub.2).sup.n .congruent.R.sub.1 /R.sub.2. (2)
This leads to the conclusion that a flaw distribution fractured at two different loading rates will give two different strength distributions in constant strength ratio to one another. The relationship expressed in equation (2) was used in the manner described hereinbelow to experimentally determine the fatigue constant n of fibers treated in accordance with the method of the present invention.
2. Prior Art Optical Fiber Strengthening Techniques
It is already known to manufacture optical fibers which are coated during manufacturing with plastic material which adds its own tensile strength to that of the fiber and also protects the fiber from external damage. Also known are methods of protecting the fiber itself from static fatigue failure. United States Defensive Publication No. T958,010 teaches that glass optical fibers can be protected by an initial coating that reacts with surface silanol groups, thereby occupying or tying up water vapor reaction sites. The OH surface radicals may be bonded to by condensation reactions including: esterification, carboxylation, etherification, chlorination and ammination.
Conventional plastic coatings are nearly worthless in retarding the diffusion of water from the atmosphere to the glass surface. Among the most successful coatings that act as hermetic seals are thick metallic coatings. However, the effectiveness of such coatings is dimensioned because bending of the fiber produces plastic flow in the metal which, in turn, causes microbending attenuation in the fiber.
Another known method of increasing the strength of glass optical fibers is to provide surface compression at the cladding surface. Such a technique is discussed, for example, in an article in the Journal of the American Ceramic Society, December 1969, pages 661-664, by D. A. Krohn et al. This article presents theoretical and experimental data to show that, if the cladding glass is selected to have a lower coefficient of thermal expansion than that of the core glass, and if proper attention is paid to glass transition temperatures of the core and cladding, there is a good probability that compressive stresses can be developed to improve fiber strength.
3. Prior Art Optical Fiber Heating Techniques
U.S. Pat. No. 3,711,262 teaches one of the first known techniques for forming low loss optical waveguide fibers, i.e., fibers having losses less than 20 dB/km. Such waveguides included a core of silica doped with an oxide such as titania. It was found that the dopant became chemically reduced during fabrication which included the step of drawing the fiber at a high temperature. For example, titanium dioxide has Ti.sup.+4 ions which are reduced to Ti.sup.+3 ions during fabrication. These reduced ions increased the attenuation of the fiber. U.S. Pat. No. 3,782,914 teaches a method of reducing the attenuation of such a fiber by heat treating the fiber at a temperature in the range of approximately 800.degree.-1000.degree. C. whereby the reduced multivalent ion is oxidized with hydroxyl ions deliberately retained in the glass and which serve as the oxidizing agent. It has since been discovered that hydroxyl ions increase the attenuation of optical waveguide fibers at certain wavelengths, and optical fibers are now formed with as low a OH content as possible. Optical waveguide fiber preforms formed by the so-called inside vapor deposition and both lateral and axial outside vapor deposition techniques are capable of forming fibers the hydroxyl ion content of which is less than 100 ppm.
Furthermore, although post-drawing heat treatments resulted in fibers having low absorption losses, such heat treatments also had the detrimental effect of weakening the fibers. As taught in U.S. Pat. No. 3,788,827, the fiber had to be subjected to surface cleaning technique such as bombarding the surface with ions having sufficient energy to remove a surface layer from the fiber. Thereafter, the fiber was heat treated before the surface could become recontaminated in order to change the oxidation state of the dopant oxide, thereby reducing the light absorption properties thereof. In accordance with the teachings of U.S. Pat. No. 3,788,827, the fiber having a portion of the surface removed was stored on a reel in a chamber to which there was connected a source of oxygen which was required for the heat treatment. After a sufficient amount of fiber was stored, heating means was activated for a period of time sufficient to oxidize impurities in the fiber and improve the light transmission properties thereof to an acceptable level.